Those familiar with the technology of sampling a hydrocarbon-containing media, such as natural gas, in a flowing pipeline recognize that there are two well-known sampling techniques:
(1) "Spot sampling" involves the taking of a sample of media at one or more points in time and assuming that the sample accurately represents the composition of the media over some defined substantially long period of time, e.g. one sample per week or once per month. The spot sampling method may thus involve simply filling a sample cylinder containing a vacuum with the media, with the decision to spot sample requiring interaction by a sampler technician. More typically, a sample cylinder may be conditioned by a series of fill and empty actions to remove all air and then finally fill the cylinder to take the samples. PA1 (2) "Composite sampling" differs from spot sampling in that in composite sampling method, the media in a flowing pipeline collects the sample media slowly over some defined long period of time. Prior to starting the composite cycle, the sample cylinder may thus be conditioned using a repeating, fill and empty method, and the sample cylinder then left at ambient pressure. The composite cycle is typically performed by a positive displacement sample pump that samples a fixed volume or "bite" of media at a rate proportional to the flow, e.g., taking a selectively sized sample "bite" once every several thousand cubic feet of flowing gas. Less accurately, a sample of a fixed size may be taken once every several minutes, and typically from once every two minutes to once every sixty minutes. In yet another composite sampling technique, a small sample slowly "leaks" to the sampler bottle using a motorized regulator. The point is that the sampling equipment, not a sampling technician, determines when a sample bite is obtained.
Both the spot sampling and composite sampling techniques have historically been performed without regard to the ambient temperature of the sampling equipment or the dew point of the gas being sampled. During the act of obtaining a sample, the gas may be brought below its dew point by coming into contact with sampling equipment that is at a temperature below the dew point of the flowing gas. In this situation, components of the sampled gas will tend to condense out and cling to the walls of the sampling equipment. Since the condensed components of the gas are commonly the heavy ends that are rich in BTU content, and since these condensed components never get into the sample cylinder because they cling to the walls of the equipment or drip back into the pipeline, the sample which is taken is biased "lean", meaning that the BTU content of the gas flow into the pipeline is actually greater than that represented by the sample.
The above deficiencies in sampling equipment have long been recognized by the American Petroleum Institute (API). API has recently enacted a sampling standard that requires heating of the sampling equipment to a temperature above the dew point of the flowing gas while the sample is being taken. Cost effective and efficient solutions to the heating requirement are formidable, since typically electrical power is not available at the sampling point. Gas is plentiful and infra-red heater solutions are possible with composite samplers because they must be kept warm over an extended period of time. The sampling speed required with spot sampling, however, makes the infra-red heater solutions inefficient because of their long start-up cycle times. A prior art sampling pump with a pressure regulator is disclosed in U.S. Pat. No. 5,498,136.
The disadvantages of the prior art are overcome by the present invention, and an improved sampling system and method particularly applicable for spot sampling is disclosed. The present invention thus overcomes many of the difficulties and shortcomings of the prior art, which have long been recognized in the industry.